High luminescence and reliable light-emitting diodes (LED) which emit light spontaneously under forward bias conditions have a variety of applications such as outdoor displays, traffic signs, optical fiber communication, automobile indicator devices, and others. Such light-emitting diodes are generally a semiconductor diode consisting of a p-n junction or a p-i-n junction formed on a semiconductor substrate by epitaxy, such as liquid phase epitaxy (LPE) and metal organic vapor phase epitaxial method (MOVPE).
Please referring to FIG. 1, a conventional light emitting diode based on a double heterostructure has an n conductivity type substrate 10. Below the substrate is an n conductivity type electrode 5. A distributed Bragg reflector layer (DBR) 20 is grown above the substrate first before growing an double heterostructure on the substrate 10. The double heterostructure includes an n conductivity type lower cladding layer 30, an undoped active layer 40 and a p conductivity type upper cladding layer 50. A p conductivity type window layer 60 with indirect or high energy band gap and high conductivity is formed on top of the double heterostructure. Above the window layer 60 is the p conductivity type electrode 70 of the light emitting diode.
In the light-emitting diode provided with the distributed Bragg reflector layer (DBR) 20 as described above, the component of the radiation generated by the active layer 40 which travels in the direction opposite to the direction from the active layer 40 toward the DBR layer 20, back toward the active layer 40 so that the reflected component is added to the component which is received by the light-emitting surface directly from the active layer 40. Thus, the DBR layer 20 improves the intensity of the light emitted from the light-emitting surface, i.e. the light-emitting efficiency of the LED.
Recently, Vertical-cavity surface-emitting lasers (VCSELs) and resonant-cavity light-emitting diodes (RCLEDs) are becoming increasingly important for a wide variety of applications due to higher luminescence efficiency, higher spectral purity and light emission intensity relative to conventional LEDs. These vertically-emitting devices, with a resonant cavity perpendicular to a surface of a semiconductor wafer on which the devices are fabricated, have many advantages over edge-emitting devices, including the possibility for wafer scale fabrication and testing, and the possibility of forming two-dimensional arrays of the vertically-emitting devices. The circular nature of the light output beams from these devices also makes them ideally suited for coupling to optical fibers as in optical interconnects for integrated circuits and other applications.
VCSELs and RCLEDs have very similar device structures comprising an active region sandwiched between a pair of mirror stacks. A semiconductor p-n or p-i-n junction is formed about the active region, and an electrical injection current is provided across the junction to generate light within the active region. Electrodes above and below the mirror stacks provide an electrical connection to the devices, with one of the electrodes generally defining a central opening for the emission of light in a direction normal to the plane of the active region.